Low update rate on robot_localization

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I am using ROS kinetic, Raspbian 9.4 (Stretch) on a Raspberry Pi 3. I am equipping a miniature vehicle (similar to the image below, image from google) for autonomous driving.

So far my I have an IMU and encoders installed, and I am attempting to fuse the sensors with robot_localization ekf. The outputs of robot localization are actually close to the expected position/orientation, but the rate is unusable low and slow to converge as a consequence. Here is the output from rostopic hz.

note@pc:~$ rostopic hz odometry/filtered
subscribed to [/odometry/filtered]
no new messages
no new messages
no new messages
no new messages
no new messages
average rate: 0.290
    min: 3.444s max: 3.444s std dev: 0.00000s window: 2
no new messages
no new messages
no new messages
average rate: 0.264
    min: 3.444s max: 4.122s std dev: 0.33911s window: 3
[..]

The publish rates for the TF transforms is higher than 50Hz (image below), and robot_localization is set up to publish the odom->base_link transform.

The same goes for the topics which robot_localization are subscribed to (below). Outputs from rqt.

The configuration file for robot_localization is as shown below. I have played with frequency and sensor_timeout parameters, but I did not experiment much with the covariance matrices, as I expect to see performance changes by modifying the covariances, and not the filter`s loop frequency (Is this assumption correct?).

      <node name="local"   pkg="robot_localization" type="ekf_localization_node">

  <param name="frequency" value="100"/>
  <param name="sensor_timeout" value="0.02"/>
  <param name="two_d_mode" value="true"/>
  <param name="map_frame" value="map"/>
  <param name="odom_frame" value="odom"/>
  <param name="base_link_frame" value="base_link"/>
  <param name="world_frame" value="odom"/>
  <param name="transform_time_offset" value="0.0"/>
  <param name="transform_timeout" value="0"/>

  <param name="imu0" value="imu/data"/>
  <param name="odom0" value="enc/pos"/>

  <rosparam param="imu0_config">
      [false, false, true,
       true,  true,  true, 
       false, false, false,
       true, true, true,
       true, true, true]
      </rosparam>

  <rosparam param="odom0_config">
      [true, true, true,
       false, false, false,
       false, false, false,
       false, false, true,
       false, false, false]
      </rosparam>

  <param name="imu0_queue_size"  value="10"/>
  <param name="odom0_queue_size" value="10"/>

  <param name="imu0_differential"  value="false"/>
  <param name="odom0_differential" value="false"/>

  <param name="imu0_relative"  value="false"/>
  <param name="odom0_relative" value="false"/>

  <param name="imu0_remove_gravitational_acceleration"  value="true"/>

  <rosparam param="initial_state">[0.0,  0.0,  0.0,
                                0.0,  0.0,  0.0,
                                0.0,  0.0,  0.0,
                                0.0,  0.0,  0.0,
                            0.0,  0.0,  0.0]</rosparam>

  <param name="publish_tf"  value="true"/>

  <param name="publish_acceleration"  value="false"/>
  <param name="print_diagnostics"  value="true"/>

  </node>

Anyone would have insight on what I am missing?/ How can I get a higher update rate from robot_localization?